One of the more important tests to a motor or generator coil is the AC Withstand test. Only this test, using AC high voltage, can truly verify the integrity of the coils as they are stressed under operating conditions. This test applies an over voltage about twice the normal line to ground operating voltage, or 2 Uo. The test voltage is held for 60 seconds. It is a simple pass/fail test, where the coil either holds the voltage or fails/arcs should there be a defect. It is a “destructive” test to defects that cannot hold the test voltage but has no harmful effects on the coil where it is not defective.
In addition to this test, Diagnostic testing is also performed, namely Tan Delta/Power Factor and Partial Discharge testing, with the instruments connecting to the winding leads or the output of the AC hipot that applies the overvoltage while the test data is recorded. A good use of these tests is for stator bars, either tested individually, several at a time, or the entire coil at once, depending on how powerful an AC hipot is available.
Selecting an AC Hipot: How much current is needed?
When AC voltage testing, the current draw can be very high when AC voltage is applied to highly capacitive loads.* The current required of the test set, mAs or amps, must be calculated to properly size the instrument. If the capacitance of the coil under test is known, the current can be calculated. If not, past test data may be available. If a lower voltage and power rated AC hipot is available, apply a few kVac and measure the current, then linearly scale it up to the necessary test voltage. The capacitance can also be calculated from the design data of the coil. Refer to the full Application Page linked from this site for more details. When using DC voltage this is not an issue, but DC voltage testing is not recommended.
Amps = 2πf CV f = frequency in Hz C = load capacitance in farads V = test voltage in volts
* When testing with DC voltage, this is not an issue. However, DC over voltage testing is not recommended for coils.
HVI Product Solutions
High Voltage, Inc. AC Dielectric Test Sets: The models shown are designed to test motors & generators with voltages rated from 2.4 kV – 25 kV. Their load capacities are based on capacitive values commonly encountered. Many more models are available in many sizes and control configurations. Please see the various available TD and PD HVI accessories by downloading our PDF!